>Is there a C++17-exclusive compiler-flag? Or linters that inform you about outdated concepts? Some way to get rid of the backwards compatible cruft when writing new things.

clang-tidy might be the closest thing you're looking for:

http://clang.llvm.org/extra/clang-tidy/

There is another language that always gets mentioned in C++ posts and that languages has a tool called clippy which does what you want. It's dreamy.